1. Field of the Invention
The present invention relates to a process for preparing amines. More specifically, it relates to a process for preparing methylamines from methanol and ammonia by which monomethylamine and dimethylamine are obtained in larger amounts than trimethylamine. Still more specifically, it relates to a crystalline morphology of a mordenite for exerting a more excellent catalytic performance among mordenite catalysts which can be used to accelerate the reaction of methanol with ammonia. The methylamines which can be obtained by the present invention are useful compounds applicable to many uses as materials for the manufacture of various kinds of solvents and intermediates for synthetic organic synthetic compounds.
2. Description of the Related Art
Methylamines, i.e., monomethylamine, dimethylamine and trimethylamine can be prepared by a method which comprises reacting methanol or a mixture of methanol and dimethyl ether with ammonia, a method which comprises the catalytic hydrogenation of prussic acid, or the like.
Thus, these methylamines can be produced as a mixture of monomethylamine, dimethylamine and trimethylamine, and they have corresponding uses, respectively. On the other hand, among these methylamines, the demand of dimethylamine and monomethylamine is particularly large, but that of trimethylamine is small under the existing circumstances. In the methylamines obtained by the reaction of methanol with ammonia in the presence of a usual amorphous silica-alumina as a catalyst, trimethylamine is a main component, and the yield of dimethylamine whose demand is large is inconveniently low. It has been disclosed that in order to overcome this disadvantage, a dehydrated crystalline aluminosilicate (zeolite) having a pore diameter of from 5 to 10 .ANG. can be used in the reaction of an alcohol having 1 to 18 carbon atoms with ammonia, and in this case, the production of the monoamine and diamine predominates over that of the triamine. Furthermore, it has also been disclosed that natural zeolites and synthetic zeolites are mentioned as some kinds of zeolites suitable for the above-mentioned reaction. It has further been disclosed that suitable examples of the natural zeolite include faujasite, analcime, clinoptilolite, ferrierite, chabazite, gmelinite, levynite, erionite and mordenite, and suitable examples of the synthetic zeolite include X type, Y type and A type (U.S. Pat. No. 3,384,667, 1968).
There have also been disclosed a method which comprises mixing methanol with ammonia in a specific ratio, and then reacting them in the presence of a catalyst such as a mordenite to obtain monomethylamine in a particularly large amount (Japanese Patent Application Laid-open No. 113747/1981), and another method in which monomethylamine is disproportionated by a crystalline aluminosilicate selected from sodium ion type mordenites to selectively obtain a large amount of dimethylamine (Japanese Patent Application Laid-open No. 46846/1981).
Disclosed have also been a method in which a natural mineral is used as the mordenite in about the same manner as in the above-mentioned U.S. Pat. No. 3,384,667 (Japanese Patent Application Laid-open No. 169444/1982), a method using, as the catalyst, a mordenite subjected to ion exchange with lanthanum ions (Japanese Patent Application Laid-open No. 49340/1983), a method using, as the catalyst, a mordenite in which the amount of ion-exchanged alkaline metal ions is limited to a specific range (Japanese Patent Application Laid-open No. 210050/1984 and U.S. Pat. No. 4,578,516), a method using a steam-treated mordenite as the catalyst (Japanese Patent Application Laid-open No. 227841/1984 and U.S. Pat. No. 4,582,936), a method using an A type zeolite containing a small amount of a binder as the catalyst (Japanese Patent Application Laid-open No. 69846/1983), and a method using a Rho type (ZK-5) zeolite as the catalyst.
When the zeolite catalyst is used in the above-mentioned manner, the production of trimethylamine can be inhibited, but there is also known a method in which for the purpose of bringing the production of trimethylamine into zero or substantially zero, a mordenite having pores modified by a CVD (chemical vapor deposition) of silicon tetrachloride is used as the catalyst Japanese Patent Application Laid-open No. 262540/1991; J. Catal., Vol. 131, p. 482 (1991); and U.S. Pat. No. 5,137,854!.
Another method has also been present in which a chabazite, an erionite, a ZK-5 or a Rho type zeolite modified by precipitating a compound of silicon, aluminum, phosphorus or boron thereon is used as the catalyst to decrease the production of trimethylamine (Japanese Patent Application Laid-open No. 254256/1986 and U.S. Pat. No. 4,683,334). Furthermore, there is also known a method in which an alcohol is reacted with ammonia in the presence of SAPO of a non-zeolite molecular sieve as the catalyst to obtain an alkylamine (Japanese Patent Application Laid-open No. 734/1990).
As described above, by using various kinds of heretofore disclosed zeolite catalysts in the reaction of methanol with ammonia, the production of trimethylamine which is in small demand can be inhibited, and that of dimethylamine which is in large demand can be increased. It is well known from old days that among the zeolites usable for this purpose, the mordenites are particularly excellent in activity for the synthesis of the methylamines U.S. Pat. No. 3,384,667, 1968; and J. Catal., Vol. 82, p. 313 (1983)!. However, in the case that the mordenites employed are those prepared by a hydrothermal synthesis method or the natural mordenites, the activity for the synthesis of the methylamines and the performance for the production inhibition of trimethylamine are fairly different among these kinds of mordenites, even if a mordenite content, an impurity content, a cation composition and the like are substantially similar and even if values obtained by a standard powder X-ray diffraction and pore diameters obtained from the adsorption isotherm of argon at a liquid argon temperature are at similar levels. This difference is not so large in the case of the synthetic mordenites, but it is particularly noticeable in the case of the natural mordenites.
U.S. Pat. No. 3,384,667 (1968) has suggested that zeolites having a pore diameter of from 5 to 10 .ANG. are used in order to predominantly obtain a monoalkylamine and a dialkylamine from an alcohol having 1 to 15 carbon atoms and ammonia, and as one of these zeolites, a natural mordenite is suitable. However, it is not disclosed at all which kind of natural mordenite is suitable. In Japanese Patent Publication No. 27335/1990, it is described that in preparing the methylamines from methanol and ammonia, a natural mordenite having an effective pore diameter of from 1 to 5 .ANG. is suitable as the catalyst, but a more detailed description is not disclosed at all therein. That is to say, with regard to characteristics of the mordenites, particularly the natural mordenites suitable for the manufacture of the methylamines from methanol and ammonia, any standards for selection have not been established, and it has been difficult to prepare the catalyst for efficiently accelerating the reaction, while sufficiently inhibiting the production of trimethylamine.